Gas-phase olefin polymerization processes are economical processes for the polymerization of olefins. Such gas-phase polymerization processes can, in particular, be carried out in gas-phase fluidized-bed reactors in which the polymer particles are kept suspended by means of an appropriate gas stream. Processes of this type are described, for example, in European patent applications EP-A-0 475 603, EP-A-0 089 691 and EP-A-0 571 826, whose contents are hereby fully incorporated by reference.
In the production of polyolefins, in order to produce different polymer grades in the same reactor, there is the need, from time to time, to change the catalyst system. Therefore with a certain frequency, depending on the flexibility required to the reactor and on the production plans, it is necessary to use a first catalyst system to produce a first polymer and, subsequently, to use a second catalyst system to produce a second polymer. This change may not involve any substantial issue when a first catalyst system and the second catalyst system are compatible with one another, i.e. when both catalyst systems can operate under substantially the same process conditions (generally temperature, pressure, amount of molar mass regulator, etc.) without substantially losing activity.
However, the change from a first catalyst system to a second catalyst system which is incompatible with the first catalyst system involves problems in ensuring an adequate continuity of production in terms of both quantity and quality of the product and has therefore been the subject of much effort.
In the present description and in the following claims, two catalyst systems are incompatible to each other if they respond in different ways to process conditions and/or monomers or any process agents employed in the process, such as to molecular weight regulators, for example hydrogen, comonomers or antistatic agents, and if, due this different responsiveness, the polymers obtained by transitioning from the first catalyst system to the second catalyst system have unacceptable properties (e.g. molecular weight and/or melt flow rate and/or melt flow ratio out of the respective target value, presence of gels and fines, insufficient environmental crack resistance) or the process productivity is unacceptably low (e.g. due to chunks or sheeting in the reactor).
This definition applies to any of the components making part of the catalyst systems and mentioned above. So, in the present description and in the following claims, two catalyst systems are incompatible to each other if at least one component of the first catalyst system is incompatible with at least one component of the second catalyst system.
For example, a single site catalyst such as a metallocene catalyst is not compatible with a Ziegler-Natta catalyst mainly because, in order to produce, for example, a polyethylene having a predetermined melt flow rate, Ziegler-Natta catalysts require operating at high hydrogen concentrations (by way of illustrative example, at a ratio of hydrogen to ethylene in the order of 1).
By way of illustrative example, single site catalysts comprise metallocene catalysts. Single-site catalysts may comprise for example compounds selected from the group of metallocenes (including cyclopentadienyl derivatives, optionally substituted with cyclic compounds), phenoxyimin derivatives, as well as neutral or charged bidentate or tridentate nitrogen ligands with 2 or 3 coordinating nitrogen atoms.
In the present description and in the following claims, the expression “metallocene catalyst” is used to indicate a catalyst component comprising at least one cyclopentadienyl transition metal complex and, generally, a compound having the following formula:Cp2MR2X2 wherein Cp is a substituted or unsubstituted cyclopentadienyl ring or derivative thereof, M is a transition metal, preferably a Group 4, 5, or 6 metal, R is a hydrocarbyl group or hydrocarboxy group having from one to twenty carbon atoms, and X is a halogen. Generally, the metallocene catalyst components referenced herein include half and full sandwich compounds having one or more bulky ligands bonded to at least one metal atom. Typical metallocene catalyst components are generally described as containing one or more bulky ligand(s) and one or more leaving group(s) bonded to at least one metal atom. For the purposes of this description and appended claims, the term “leaving group” is any ligand that can be abstracted from a bulky ligand metallocene catalyst to form a metallocene catalyst cation capable of polymerizing one or more olefins.
The bulky ligands are generally represented by one or more open or fused ring(s) or ring system(s) or a combination thereof. These ring(s) or ring system(s) are typically composed of atoms selected from Groups 13 to 16 atoms, preferably the atoms are selected from the group consisting of carbon, nitrogen, oxygen, silicon, sulfur, phosphorous, boron and aluminum or a combination thereof. Most preferably the ring(s) or ring system(s) are composed of carbon atoms such as but not limited to those cyclopentadienyl ligands or cyclopentadienyl-type ligand structures or other similar functioning ligand structure such as a pentadiene, a cyclooctatetraendiyl or an imide ligand. The metal atom is preferably selected from Groups 3 through 16 and the lanthanide or actinide series of the Periodic Table of Elements. Preferably the metal is a transition metal from Groups 4 through 12, more preferably 4, 5 and 6, and most preferably the metal is from Group 4.
Single site catalysts, however, such as for example metallocene catalysts, must be operated at low hydrogen concentrations (of some centimol %, for example in the order of 0.06 mol %).
So, if Ziegler-Natta catalysts are operated at low hydrogen, they produce very high molecular weight polymers, while if metallocene catalysts are operated at low hydrogen, they produce low molecular weight polymers. Accordingly, combining the Ziegler-Natta catalyst and the metallocene catalyst and operating at low hydrogen concentration will lead to a polymer containing ultra-high molecular weight chains which, following processing, give rise to gels.
In order to perform the above-mentioned change of catalyst system, the most common method of the state of the art is that of stopping the first polymerization reaction by means of a deactivating agent, emptying the reactor, cleaning it and starting it up again by introducing the second catalyst system. Thus, for example, WO 00/58377 discloses a discontinuous method for changing between two incompatible catalysts, in which the first polymerization reaction is stopped, the polymer is removed from the reactor, the reactor is rapidly purged with nitrogen, a new seedbed of polymer particulates is introduced into the reactor and the second polymerization reaction is then started. However, on the one side the opening of the reactor leads to deposits on the walls which have an adverse effect on the renewed start-up of the reactor and, on the other side, such method inevitably requires a discontinuation of the polymerization process and an unacceptably long stop time between the first polymerization reaction and the second polymerization reaction.
Application WO95/26370 describes a process for transitioning from a polymerization reaction catalyzed by a first catalyst system to a polymerization reaction catalyzed by a second catalyst system comprising a metallocene catalyst, wherein the first and second catalyst systems are incompatible. According to WO95/26370, the introduction of the first catalyst system into the reactor is discontinued, an irreversible catalyst killer and optionally a reversible catalyst killer is (are) introduced in the reactor, and then the second catalyst system is introduced into the reactor. Although among the catalyst systems described by WO95/26370, mixed catalyst systems comprising a metallocene catalyst are generally envisaged, there is no specific teaching on the transitioning from a first catalyst system to a second catalyst system of the mixed type.
WO2007/059867 describes a method of changing from a polymerization using a first catalyst to a polymerization using a second catalyst which is incompatible with the first catalyst in a gas-phase reactor, which comprises the steps of stopping of the polymerization reaction using the first catalyst, flushing of the reactor under polymerization conditions with at least one deactivating agent, introducing the second catalyst into the reactor, and continuing the polymerization using the second catalyst. The second catalyst may be a mixed catalyst. Although a generic reference to the possibility of retaining the particle bed is made, still the teaching of WO2007/059867 is that of emptying the reactor and filling it with a new particle bed.
Furthermore, even in view of the teaching of these prior art documents, there is still the need to reduce the transition time used to perform the above-mentioned transitioning.